This invention generally relates to error correction for data transmitted in a network, and particularly relates to facilitating error correction of frames carried over a network.
Given the ever-increasing demands for voice, data, and multimedia communications, corporations are continuing to take advantage of the high-speed digital communications of the T Carrier systems. The most prominent carrier system is the T1. A T1 describes the physical layer interface to a provider network. Once the T1 carrier is in place and terminated, a customer may generate traffic. The digital signal transmitted over a T1 is referred to as digital signal level one (DS1). DS1 operates at a digital signaling rate of 1.544 Mbps.
DS1 traffic is arranged in fixed-length frames, wherein each frame consists of 193 bits of information created in 125 xcexcseconds. One bit is used for framing and the other 192 bits are used for customer traffic. The 192 bits could be data or 24 eight-bit voice channels. Since one frame is created every 125 xcexcseconds, there are 8,000 frames per second, which results in the desired 1.544 Mbps data rate. Typical DS1 framing and formatting is shown in FIG. 1.
The key quality measure for DS1 service is frame error rate. The consequences of frame errors are loss of quality for certain applications like voice or video, or loss of efficiency due to the application requiring retransmission of the data. In the past, DS1 traffic was carried entirely over traditional time division multiplexing (TDM) networks. There is now a movement to carry DS1 traffic over ATM (Asynchronous Transfer Mode) networks. ATM is a connection-oriented, packet-switching network technology that uses fixed-size cells to carry data. ATM requires that all cells be the same size to enable faster switching and relay across ATM switches, which make up the ATM network.
Each ATM cell is 53 octets long, including a 48-octet payload preceded by a 5-octet header. Notably, other octets of the 48-octet payload may be used for the ATM adaption layer, which facilitates mapping data into and extracting data from a cell. For example, AAL1 (ATM Adaption Layer 1) has been commonly used to carry circuit-switched voice information. An emerging AAL1 use is circuit emulation for carrying DS1. AAL1 uses one octet out of the 48-octet payload.
When carrying DS1 traffic over an ATM network, the fixed DS1 frames of 193 bits do not map directly into the 47 octets (376 bits) remaining in the payload of an ATM cell. When DS1 frames are consecutively mapped into the payload of ATM cells, the error rate for transport increases substantially over that normally associated with transporting DS1 frames over a TDM network. There are several reasons for this increased error rate. First, the use of ATM requires additional data manipulation, which may cause error. This manipulation includes mapping of all or part of a DS1 frame into an ATM cell, removing the DS1 frame or portion thereof from the ATM cell, and reassembling DS1 frames for transport over a final DS1-compatible carrier.
Second, carrying DS1 frames over an ATM network requires the addition of information in the form of a header, which must be communicated accurately for proper transport of the DS1 frame. Third, corruption of any single ATM cell likely leads to the corruption or loss of multiple DS1 frames having portions carried within the corrupted cell. Finally, ATM networks, unlike TDM networks, will lose DS1 frames due to lost ATM cells caused by traffic congestion.
Corruption and loss of ATM cells have a significant impact on error rates and efficiency. If a cell is lost or corrupted, DS1 frame errors will occur. Network designers typically try to maximize efficiency by packing as much information into each cell as possible. Because of this, a direct mapping of DS1 frames into ATM cells will include at least portions of two or three DS1 frames. Thus, loss of one cell results in up to three DS1 frame errors.
A direct mapping of DS1 frames into ATM cells is shown in FIGS. 3A and 3B. The first number in each cell represents the number of bits from a partial DS1 frame carried in a previous ATM cell. The second number indicates that either a full 193-bit DS1 frame was mapped in the cell or no full DS1 frame was placed in the cell. The third number indicates the number of bits of a partial frame, the remainder of which will be mapped in a subsequent ATM cell.
When carrying DS1 over an ATM network in this manner, the ATM network will multiply the error rate normally occurring over TDM networks. For direct mapping, the frame loss due to bit errors for DS1 traffic will be on the order of 3.3 times (or more) worse when carried over ATM. If cell loss due to traffic is also considered, then DS1 frame loss will be orders of magnitude greater than that of a standard TDM network.
Therefore, there is a need to provide efficient transmission of DS1 traffic over an ATM network without increasing the error rate normally associated with DS1 traffic carried over a traditional TDM network.
The present invention provides for efficient transmission of DS1 traffic over an ATM network without increasing the frame error rate normally associated with DS1 traffic. These improvements are achieved by inserting into each ATM cell payload information from one or more DS1 frames and error correction bits for an error correction code. The error correction code is configured to facilitate error correction associated with the transport of DS1 frames over the ATM network. Error correction is provided when the DS1 frames are extracted from the ATM cells for final transport over another TDM network.
The error correction code may correct errors in a portion of a DS1 frame, an entire DS1 frame, a portion of an ATM cell, or an entire ATM cell. Further, the error correction code may correspond to the information within the cell in which the error correction code is transported or may relate only to other ATM cells. The extent and robustness of error correction is variable based on need.
In general, creating and placing an error correction code in a cell for error correction of data or frames carried by that cell or related cells is applicable for various network technologies. When frames of data arrive at a gateway between two networks, the frames of data are mapped into cells used for transport. Error correction codes are generated and added into the payload of each transport frame or cell. The transport cells encapsulating the data frames and error correction code are carried over the network to another gateway, wherein the transport cells are processed and corrected for errors based on an error correction code in the transport cells.
Providing the error correction code allows for the correction of lost or corrupted information during transport without requiring retransmission of data. When multiple frames of data are mapped into a transport cell, error correction for one transport cell will eliminate multiple frame errors. Other aspects and features of the present invention will become apparent to those skilled in the art.